add custom texts(labels or annotations) which are always upright to user in view port
Labels in Render
An easy (but limited) way would be to parent the text labels to the camera.

Select text label RMB
Add camera to selection Shift-RMB
Set camara to parent Ctrl-P


After that movement of the camera or transforming the objects in the scene would no longer influence the appearance of the label.
Example rotating object and moving camera:

Labels in Viewport
If you want to draw labels in the view port you find all you need in Adhi's display_matrix.py script.